Charge pumps are used to generate voltage levels (positive or negative) outside of the supply voltage rail levels. These higher than normal voltages are generated in a charge pump unit cell by dumping electric charge through the use of capacitors driven by clock driver circuits. Charge pumps typically have several stages of this charge pump unit cell to attain a particular high voltage. A feedback regulation scheme around the charge pump circuit makes sure the final output voltage is at the required level.
A conventional charge pump circuit uses a fixed value capacitor in the charge pump circuit. Conventional charge pumps have a tradeoff between low ripple and fast turn-on. For many charge pumps the turn-on time is not important, because the circuit is almost always on. However, in power saving applications it is necessary to power down the charge pump circuit often. As a result, the speed of the turn-on time becomes critical. Convention charge pumps either have a fast turn-on time or a low ripple for a given clock frequency and output load, but not both.
Thus there exists a need for a charge pump circuit that has both a fast turn-on time and a low voltage ripple at operating voltages for a given clock frequency and output load.